Process for the preparation of acrylic acid



Patented Jan. 18, 1949 PROCESS FOR THE PREPARATION OF C ACRYLIC ACID Seaver A. Ballard, Orinda, and Bradford P. Geyer, Berkeley, Calif., assignors to Shell Development Company, San Francisco, Calih, a corporation of Delaware No Drawing.

Application February 15, 1947, Serial No. 128,961

4 Claims. (CL 260-526) This invention relates to a method of preparing acrylic acid.

One object of the invention is an improved method of preparing acrylic acid particularly by a process comprising pyrolytic treatment of cer-- tain beta-acyloxy propionic acids. A further object of the invention comprises the provision of particular conditions whereunder such pyrolytic treatment may be effected with maximum advantage in the nature of the results obtained. A further and related object comprises a process whereby acrylic acid may be prepared from acrolein in an improved manner by a novel combination of procedural steps, as described hereinafter. Other objects of the invention will become apparent as the invention is more fully described herein. I

The process which is the subject of the present invention comprises subjecting to pyrolytic treat-f ment at a suitable elevated temperature a beta: acyloxy propionic acid to effect its thermal decomposition into acrylic acid and the carboxylic acid from which the substituent acyloxy group is derived. In accordance with the present invention, the free beta-acyloxy propionic acid is subjected to pyrolytic treatment. Possible undesired side reactions such as decarboxylation, cyclization, de-

hydration, etc., which have been observed in the most convenient to employ a beta-acyloxy propionic acid in which the acyloxy group is derived from an aliphatic carboxylic acid having a boiling point that differs from the boiling point of acrylic acid by an amount sumcient to permit ready separation of the two carboxylic acids by fractional distillation, say by a diflerence of at least 10 C. If it is not desired to separate the individual carboxylic acids formed by the reaction upon which the present process is based, the acyloxy substituent group may correspond to a carboxyllc acid having a boiling pointonly slightly different from or even the same as that of acrylic acid. It is preferred to employ the open chain, aliphatic beta-acyloxy propionic acids which contain only saturated carbon-to-carbon bonds; 1. e., the betaacyloxy propionic acids wherein the substituent acyloxy group is derived from a saturated fatty acid such as a lower saturated fatty acid containing up to eight carbon atoms. Beta-acyloxy propionic acids within this preferred class include, for example, betaeacetoxypropionic acid, beta-propionoxypropionlc acid, beta-butyroxypropionic acid,

beta-isobutyroxypropionic acid, beta-valeroxypropionic acid, beta-isovaleroxypropionic acid, etc.

These and other-beta-acyloxy propionic acids maybe prepared in various ways. For example,

case of pyrolytic treatments of other carboxylic materials, are substantially avoided. Under the conditions utilized in accordance with the present process, highly satisfactory yields of acrylic acid are obtained. The substantial absence of such possible side reactions, and the high yields of acrylic acid were unexpected results of the present process, particularly in view of the dimculty of predlcting'the probable course of pyrolytic reactions in the case of complex materials such as those herein employed. As a consequence of these and similar results of the present process, therehas been provided a novel and improved method of preparing acrylic acid that requires a minimum of equipment and that offers numerous advantages from the standpoint of execution on a commercial scale.

The beta-acyloxy propionic acids that are employed in the execution of the process of the present invention may be referred to by the structural formula Rcoocmcmcoon present, are linked together by saturated and/or."

aromatic carbcn-to-carbon bonds. It generally is in certain cases they may be prepared by treatment of beta-hydroxypropionic acid with a suitable esterifying agent, such as the acid anhydride or acid halide corresponding to the substituent acyloxy group that it is desired to introduce. In other instances, they may be prepared advantageously by treatment of a beta-halo propionic acid with, for example, a silver or other metal salt of the carboxylic acid corresponding to the acyloxy substituent group to be introduced. Although these, and other methods that are available and that will be apparent to those skilled in the art, provide numerous possible ways of preutilized in the process of the present invention,

a particularly convenient method for their preparation comprises utilizing acrolein as a raw material for their synthesis. By means of the process disclosed in the copending application, Serial No. 728,962, filed February 15, 1947, acrolein thus may be reacted with a carboxylic acid such as a lower fatty acid to form, by addition of the acid to the alpha and the beta carbon atoms of the acrolein, the corresponding beta-acyloxy propionaldehyde. The beta-acyloxy propionaldehyde then may be oxidized in any suitable manner to the corresponding beta-acyloxy propionic acid, for example, by treating the beta-acyloxy propional- 3 dehyde with gaseous oxygen or an oxygen-containing gas such as air, in the presence of a catalytically active compound of a metal, e. g., a cobalt salt such as cobalt acetylacetonate, this method being described in the copending application, Serial No. 728,960, filed February 15, 1947, with particular reference to the oxidation of betaacetoxypropionaldehyde to beta-acetoxypropionic acid.

The reaction between the acrolein and the carboxylic acid may be effected by heating a mixture of acrolein with a molar excess of the carboxylic acid, e. g., from about 2 to about 6 moles of the carboxylic acid per mole of acrolein, to a reaction temperature of from about 50 C. to

about 160 C., under pressure sufllcient to maintain the reaction mixture in the liquid state and for a time sufficient to effect appreciable formation of the beta-acyloxy propionaldehyde. The process desirably is executed in the absence of any added material having catalytic activity. The presence of a polymerization inhibitor such as hydroquinone is not excluded, however. After the mixture has been maintained at the reaction temperature for a suitable time, say after a reaction time of from about 2 to about 12 hours, the reaction mixture may be subjected to a suitable separation treatment such as fractional distillation to recover the resultant beta-acyloxy propionaldehyde and, if desired, to reclaim any excess or unreacted reactants for reutilization in the process.

The beta-acyloxypropionaldehyde thus obtained may be oxidized to the corresponding betaacyloxy propionic acid with particular advantage according to the method disclosed in the aforesaid copending application, Serial No. 728,960. According to this method, the beta-acyloxy propionaldehyde dissolved in a suitable substantially anhydrous acidic organic solvent such as acetic acid, propionic acid, etc., or mixtures thereof with other organic solvents, e. g., a hydrocarbon solvent, an ether or an ester, is treated with oxygen or an oxygen-containing gas in the presence of a catalytically active salt of a metal such as a catalytically active salt of a metal of group VII or VIII of the periodic table of the elements. Particularly effective results may be obtained by utilizing a cobalt chelate salt of a beta-diketone, such as cobalt acetylacetonate. The catalyst is employed in catalytically eflective amounts, such as from about 0.5 to about 15 per cent by weight of the beta-acyloxypropionaldehyde. Reaction temperatures of from about 40 C. to about 90 C. are eminently satisfactory for effecting the oxidation reaction. After completion of the oxidation reaction, the catalyst may be removed if desired from the reaction medium as by precipitation or otherwise, and the beta-acyloxy propionic acid recovered in any effective manner, for example, by fractional distillation.

.At least in theory, it would appear that direct oxidation of acrolein to acrylic acid would ofler a highly economical and convenient route from this particular raw material to acrylic acid. In actual practice, the direct oxidation of acrolein to acrylic acid has been attended with considerable difficulty. Oxidative treatment of acrolein appears to be conducive to the formation, as products of side reaction, of materials which are highly active in promoting polymerization of either acrolein or the acrylic acid to products of relatively high molecular weight. The present meth od of preparing acrylic acid from acrolein avoids in substantial measure these and similar difficulties heretofore encountered. It will be noted that by utilizing the combination of steps herein described there is obtained a maximum of economy relative to reactants required, and similar advantages, since (a) the carboxylic acid utilized for reaction with the acrolein is recovered in the final step of the process and hence may be reutilized, (b) no organic reactants other than this carboxylic acid and the acrolein are required (0) no catalysts, other than the metal salt oxidation catalyst, are required and (d) air may be used as the oxidizing agent in the second step of the process with desirable savings in cost.

When the foregoing or an equivalent combination of steps is employed to prepare acrylic acid from acrolein via a beta-acyloxy propionaldehyde. it is in general most convenient and economical to employ acetic acid as the carboxylic acid. In a preferred embodiment of the invention, acrolein thus is reacted with acetic acid to obtain beta-acetoxypropionaldehyde, the betaacetoxypropionaldehyde is oxidized to beta-acetoxypropionic acid, and the beta-acetoxypropionic acid is subjected to pyrolytic decomposition whereby acrylic acid and acetic acid are obtained as the principal products of the pyrolytic decomposition.

The reactions which are involved in the conversion of acrolein to acrylic acid according to the invention may be illustrated by the following equations. In the equations, acetic acid is an exemplary lower saturated fatty acid. The first step of the process is eliminating the carbon-tocarbon unsaturation of the acrolein by addition of a molecule of acetic acid at the olefinic bond, according to the equation A 01130 O OH+CHz=C H-CHO CHzC OO-CHaCHr-CHO In the second reaction of the process, the product is converted to the corresponding carboxylic acid by oxidizing the formyl group to carboxyl, according to the equation The molecule of acetic then is removed from the product of the oxidation by pyrolysis to form the desired acrylic acid according to the equation acid in either the liquid phase or the vapor phase to effect its pyrolytic decomposition into the component carboxylic acids, that is, according to the equation in which R has its previous significance. It has been found that temperatures above about C. are most effective for promoting the pyrolytic decomposition according to the foregoing equation. Temperatures below about 170' C. have been found to be generally ineflective. The temperature of reaction may be considerably higher than 170 C. if desired, provided temperatures which favor substantial decomposition of either or both of the component carboxylic acids are avoided. Temperatures of from about 170 C. to about 300 C., may be employed. A preferred range is from about 170 C. to about 225 C., although in any given instance the most effective temperature depends to a certain extent upon the duration of heating, the particular betaacyloxy propionic acid utilized, and similar factors. In general, pressures above, below, or equalto atmospheric pressures may be used. Substantially atmospheric pressures are as a rule invention, the process is effected as a liquid phase process. The beta-acyloxy propionic acid thus may be heated either intermittently, in a batchwise manner, or continuously, for a period of time and at an elevated temperature suilicient to effect its appreciable decomposition into the component carboxylic acids. The acrylic acid then may be separated, if desired, from the resultant mixture in any suitable manner. In the case of liquid phase operation, the pressure desirably is sufficiently high to prevent substantial volatilization of the beta-acyloxy propionic acid. Atmospheric pressures are generally suitable although higher pressures may be employed, if desired, to enable the use of higher pyrolysis temperatures than would be practicable under atmospheric pressures.

The time of heating may be varied according to the other conditions employed and may be sufllcient to result in either partial or complete decomposition of the beta-acyloxy propionic acid into its component carboxylic acids. The severity of the pyrolytic treatment in any given case determines the extent of the pyrolytic decomposition of the beta-acyloxypropionic acid, and is erably under reduced pressures, and subjecting the vapors to pyrolytic conditions of temperature and time, either batchwise, intermittently or. preferably, continuously. The gaseous betaacyloxy propionic acid thus may be passed through a heated tube maintained at from about 170 C. to about 300 C. or higher at a rate providing a suitable contact time, and the gaseous in turn determined by the correlated conditions of reaction temperature and reaction time. For any given extent of pyrolysis, the use of lower reaction temperatures entails the use of longer reaction times, and vice versa. It is not essential that the beta-acyloxy acid be completely pyrolyzed in any single treatment, although it usually is more economical to efiect the process under such conditions that a relatively high percentage is reacted. At any given temperature, a relatively wide range of reaction times may be employed. However, reaction times of from about 5 minutes to about 60 minutes are particularly suitable when the process is efiected as a liquid phase process at temperatures between about 170 C. and about 225 C. In an intermittent or batchwise type of process, the betaacyloxy propionic acid may, for example, be heated under atmospheric or other pressure to a temperature and for a time sufiicient to efiect its decomposition at least in part. The pressure then may be lowered sufiiciently to remove by distillation the component carboxylic acids thus formed. The residual, undecomposed betaacyloxy propionic acid then may be further treated, if desired with replenishment of the portion that was decomposed, at elevated temperatures and pressures adapted to effect its de-.

composition. In the case of continuous operations in the liquid phase, a stream of the betaacyloxy carboxylic acid may be passed through a suitable heated reaction zone such as a heated reaction tube, and the carboxylic acids formed by the pyrolytic decomposition separated therefrom. The process may be carried out under such conditions that simultaneous pyrolysis of the beta-acyloxy propionic acid and distillation of the component carboxylic acids occurs. Any undecomposed beta-acyloxy carboxylic acid may be recycled through the process if desired.

The pyrolytic decomposition ,of the betaacyloxy propionic acid may be effected in the vapor phase as by volatilizing a volatile betaacyloxy propionic acid of the present class, prefmixture leaving the tube treated as by fractional condensation or otherwise to separate and to recover the acrylic acid from the reaction mixture. The tube may be either otherwise empty or, more desirably, it may contain inert or catalytically active solid contact materials adapted to increase the contact surface in the reaction zone. Glass, quartz, carbon, metals such as iron, copper, steel, nickel, aluminum, etc., all may be employed satisfactorily as the contact material. The contact material may be either in the form of discrete particles, fragments, pellets, and the like, or as rods, screens, filaments. etc. Reaction promoting catalysts may be employed, if desired,

although the reaction more desirably is effected in the absence of any added catalyst. An inert diluent gas, such as nitrogen, carbon dioxide, etc.,

ployed, although contact times of not over about 25 seconds generally are adequate. The term contact time as used herein may be defined as the reciprocal of the relative volume 0! vapors under the existing conditions of temperature and pressure passed through a unit volume of reaction space per second. Lower reaction temperatures generally require-the use oflonger contact times, and vice versa. By suitably correlating the contact time and the temperature, a variety of conditions may be employed eflective- 1y. i

The present process may be effected advantageously in liquid-vapor phase, as by passing a liquid stream Of the beta-acyloxy propionic acid into contact with a heated surface or into a heated reaction tube, maintained at a temperature and pressure adapted simultaneously to substantially volatilize and decompose pyrolytically the beta-acyloxy propionic acid.

In the execution of the present process, it

' generally is desirable to provide the presence of merization, etc. Such inhibitors may be added directly to the beta-acyloxy propionic acid and desirably are'added to the crude and the more highly purified acrylic acid produced by the present process. The customary amounts, say

7 from 0.5 to about 5 per cent, are satisfactory.

,- Ii desired, the present process may be executed in the presence of suitable catalysts such aszinc' chloride, calcium chloride, etc. However the process has been found taproceed very effectively in the absence of catalytic materials. Since such materials may promote various side reactions with resultant decreased yield of the desired products of reaction, it generally is preferred to effect the pyrolysis by thermal-means only, i. e., in the absence of added catalytic agents, etc.

' The reaction mixture resulting from the present process may be separated into its several ingredients in any suitable manner. Preferably, the acyloxy group of the beta-acyloxy acid that is employed, is one derived from a volatile acid, in which case separation of the several carboxylic acids may be effected most readily by fractional distillation. Other modes of separation may be employed in suitable cases, however, such as chemical separations as by conversion of the free acids to derivatives such as salts and separation of the salts by crystallization or other means.

The following examples will illustrate certain embodiments of the present invention. It will be appreciated however that the invention in its broader aspects is not intended to be limited according to the type of apparatus used, the particular reaction conditions set forth in the examples, or otherwise except as it is defined in the appended claims.

Example I Substantially anhydrous beta-acetoxypropionic acid containing about 1 per cent of hydroquinone washeated under atmospheric pressure to 186 C.v

- decomposed. Upon redlstillation in the presence of hyroquinone of the mixed acids thus accumulated, acrylic acid was recovered in a yield of 77% based on the beta-acetoxypropionic acid consumed. The neutralization equivalent of the acrylic acid thus prepared was found to agree with the theoretical value for acrylic acid within experimental error.

Example I] A liquid steam of beta-acetoxypropionic acid contained 1 per cent of hydroquinone was passed under a pressure of 50 to 60 pounds per square inch (gauge) through a stainless steel reaction tube maintained at 190 C. The mixture leaving the tube was continuously distilled under a pressure of 25 millimeters of mercury to remove acrylic acid and acetic acid, and the remaining, undistilled beta-acetoxypropionic acid was recycled through the process. The crude mixture of acetic and acrylic acids was fractionated by further distillation in the presence of hydroquinone whereby acrylic acid was recovered in more highly purified form and in good yield.

Example 111 A mixture of 100 parts of acrolein and 482 parts of glacial acetic acid was heated in a closed, glasslined reaction vessel for four hours at 120 C. Unreacted acrolein was distilled from the resultant mixture. One part of cobalt acetylacetonate dissolved in a small amount of glacial acetic acid was added to the undistilled part of the mixture, and a stream of air was passed through the resultant mixture while the temperature was maintained at about 60 C. by cooling coils immersed in the reaction mixture. When the reaction was completed, as indicated by a drop in the temperature of the reaction mixture, the excess acetic acid was removed by distillation under reduced pressure. A fraction consisting essentially of 'beta-acetoxypropionic acid thereafter was sepatube employed in Example II, at a pressure of 50 to 60 pounds per square inch and 'at C., at a velocity corresponding to a contact time of 20 seconds. Acrylic acid was recovered in good yield by fractional condensation of the gaseous mixture leaving the reaction tube.

We claim as our invention:

1. The cyclic process for converting acrolein to acrylic acid which comprises, eliminating carbonto-carbon unsaturation of the acrolein by'a'dding a molecule of acetic acid thereto at the olefinic bond, oxidizing the product of the addition to convert the formyl group thereof to carboxyl by treatment with molecular oxygen in the presence of an oxidation catalyst, and then removing the molecule of acetic acid from the oxidation product by pyrolysis thereof to obtain acrylic acid and returning the acetic acid to the first-stated step of the process.

2. The cyclic process for converting acrolein to acrylic acid which comprises adding a molecule of acetic acid to the olefinic carbon atoms by reactin the acrolein with acetic acid to form beta acetoxypropionaldehyde, converting the beta-acetoxypropionaidehyde to the corresponding carboxylic acid by oxidizing the formyl group thereof to carboxyl, and then removing the molecule of acetic acid from the product'by pyrolysis thereof to form acrylic acid and returning the acetic acid to the first-stated step of the process.

3. The process for converting acrolein to acrylic acid which comprises, eliminating carbon-tocarbon unsaturation of the acrolein by reacting acrolein with a lower saturated fatty acid to form a beta-acyloxypropionaldehyde wherein the acyl group is the acyl residue of the saturated fatty acid, converting the beta-acyloxypropionaldehyde to the corresponding carboxylic acid by oxidizing the formyl group to carboxyl, and pyrolyzing the product to remove the molecule of the lower saturated fatty acid and thereby form acrylic acid.

4. The process according to claim 3 when the lower saturated fatty acid is acetic acid.

SEAVER A. BALLARD. BRADFORD P. GEYER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES rams Number Name Date 2,093,519 Harmann Sept. 21, 1937 2,265,814 Ritchie et ai. D80. 9, 1941 2,237,537 5111.112 June 23, 1942 2,361,036 Kung Oct. 24, 1944 2,402,129 Filachlone June 18, 1946 OTHER REFERENCES Burns et al., J. Chem. Soc. (London), part I.

Jan-June, 1935, pages 400-406.

Shorguin et al., Chemical Abstracts, vol. 29, col. 7942, 1935.

Sherlin et al., Chemical Abstracts, vol. 32, col. 5398, 1938. 

